6.4

6.4

Electrophilic Addition of

Electrophilic Addition of

Hydrogen Halides to

Hydrogen Halides to

Alkenes

Alkenes

+

+

E

E

—

—

Y

Y

δ+

δ+

δ

δ

–

–

C

C

C

C

E

E

Y

Y

C

C

C

C

General equation for electrophilic addition

General equation for electrophilic addition

General equation for electrophilic addition

+

+

H

H

—

—

X

X

δ+

δ+

δ

δ

–

–

C

C

C

C

H

H

X

X

C

C

C

C

When EY is a hydrogen halide

When EY is a hydrogen halide

When EY is a hydrogen halide

CH

CH

3

3

CH

CH

2

2

CH

CH

2

2

CH

CH

3

3

H

H

H

H

CH

CH

3

3

CH

CH

2

2

CH

CH

2

2

CHCH

CHCH

2

2

CH

CH

3

3

Br

Br

(76%)

(76%)

CHCl

CHCl

3

3

,

,

-

-

30°C

30°C

C

C

C

C

Example

Example

Example

HBr

HBr

Electrophilic addition of hydrogen halides

Electrophilic addition of hydrogen halides

to alkenes proceeds by rate

to alkenes proceeds by rate

-

-

determining

determining

formation of a carbocation intermediate.

formation of a carbocation intermediate.

Mechanism

Mechanism

Mechanism

Electrons flow from

Electrons flow from

the

the

π

π

system of the

system of the

alkene (electron rich)

alkene (electron rich)

toward the positively

toward the positively

polarized proton of

polarized proton of

the hydrogen halide.

the hydrogen halide.

Mechanism

Mechanism

Mechanism

X

X

H

H

H

H

C

C

C

C

Mechanism

Mechanism

Mechanism

..

..

..

..

:

:

C

C

C

C

+

+

..

..

..

..

:

:

X

X

:

:

–

–

X

X

H

H

H

H

C

C

C

C

Mechanism

Mechanism

Mechanism

..

..

..

..

:

:

C

C

C

C

+

+

..

..

..

..

:

:

X

X

:

:

–

–

H

H

C

C

C

C

..

..

..

..

X

X

:

:

6.5

6.5

Regioselectivity of

Regioselectivity of

Hydrogen Halide Addition:

Hydrogen Halide Addition:

Markovnikov's Rule

Markovnikov's Rule

When an unsymmetrically substituted

When an unsymmetrically substituted

alkene reacts with a hydrogen halide,

alkene reacts with a hydrogen halide,

the hydrogen adds to the carbon that

the hydrogen adds to the carbon that

has the greater number of hydrogen

has the greater number of hydrogen

substituents, and the halogen adds to

substituents, and the halogen adds to

the carbon that has the fewer hydrogen

the carbon that has the fewer hydrogen

substituents.

substituents.

Markovnikov's Rule

Markovnikov's Rule

acetic acid

acetic acid

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Example 1

Example 1

Example 1

Markovnikov's Rule

Markovnikov's Rule

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

HBr

HBr

(80%)

(80%)

CH

CH

3

3

CH

CH

3

3

CH

CH

3

3

C

C

Br

Br

(90%)

(90%)

C

C

C

C

Markovnikov's Rule

Markovnikov's Rule

Example 2

Example 2

Example 2

acetic acid

acetic acid

HBr

HBr

CH

CH

3

3

CH

CH

3

3

H

H

H

H

0°C

0°C

CH

CH

3

3

Cl

Cl

CH

CH

3

3

(100%)

(100%)

Markovnikov's Rule

Markovnikov's Rule

Example 3

Example 3

Example 3

HCl

HCl

6.6

6.6

Mechanistic Basis

Mechanistic Basis

for

for

Markovnikov's Rule

Markovnikov's Rule

Protonation of double bond occurs in

Protonation of double bond occurs in

direction that gives more stable of two

direction that gives more stable of two

possible carbocations.

possible carbocations.

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Mechanistic Basis for Markovnikov's Rule:

Mechanistic Basis for Markovnikov's Rule:

Example 1

Example 1

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

acetic acid

acetic acid

HBr

HBr

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Mechanistic Basis for Markovnikov's Rule:

Mechanistic Basis for Markovnikov's Rule:

Example 1

Example 1

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

HBr

HBr

CH

CH

3

3

CH

CH

2

2

CH

CH

—

—

CH

CH

3

3

+ Br

+ Br

–

–

+

+

Br

Br

CH

CH

3

3

CH

CH

2

2

CHCH

CHCH

3

3

Mechanistic Basis for Markovnikov's Rule:

Mechanistic Basis for Markovnikov's Rule:

Example 1

Example 1

CH

CH

2

2

CH

CH

3

3

CH

CH

2

2

CH

CH

HBr

HBr

CH

CH

3

3

CH

CH

2

2

CH

CH

—

—

CH

CH

3

3

+ Br

+ Br

–

–

+

+

CH

CH

3

3

CH

CH

2

2

CH

CH

2

2

—

—

CH

CH

2

2

+

+

primary carbocation is less stable: not formed

primary carbocation is less stable: not formed

Cl

Cl

CH

CH

3

3

CH

CH

3

3

H

H

Mechanistic Basis for

Mechanistic Basis for

Markovnikov's Rule:

Markovnikov's Rule:

Example 3

Example 3

0°C

0°C

HCl

HCl

Cl

Cl

CH

CH

3

3

CH

CH

3

3

H

H

CH

CH

3

3

H

H

H

H

+

+

Cl

Cl

–

–

Mechanistic Basis for

Mechanistic Basis for

Markovnikov's Rule:

Markovnikov's Rule:

Example 3

Example 3

HCl

HCl

Cl

Cl

CH

CH

3

3

CH

CH

3

3

H

H

CH

CH

3

3

H

H

H

H

+

+

+

+

H

H

CH

CH

3

3

H

H

Cl

Cl

–

–

Mechanistic Basis for

Mechanistic Basis for

Markovnikov's Rule:

Markovnikov's Rule:

Example 3

Example 3

HCl

HCl

secondary

secondary

carbocation is

carbocation is

less stable:

less stable:

not formed

not formed

6.7

6.7

Carbocation Rearrangements in

Carbocation Rearrangements in

Hydrogen Halide Addition

Hydrogen Halide Addition

to Alkenes

to Alkenes

HCl, 0°C

HCl, 0°C

CH

CH

3

3

CHC

CHC

H

H

(CH

(CH

3

3

)

)

2

2

+

+

CH

CH

3

3

CHC

CHC

H

H

(CH

(CH

3

3

)

)

2

2

Cl

Cl

(40%)

(40%)

CH

CH

3

3

CHC(CH

CHC(CH

3

3

)

)

2

2

+

+

H

H

CH

CH

3

3

CH

CH

2

2

C(CH

C(CH

3

3

)

)

2

2

Cl

Cl

(60%)

(60%)

Rearrangements sometimes occur

Rearrangements sometimes occur

Rearrangements sometimes occur

H

H

2

2

C

C

CHC

CHC

H

H

(CH

(CH

3

3

)

)

2

2